For monitoring a three-phase high-voltage transformer by measuring quantities correlated with the loss factor, several solutions are known in the prior art, described briefly below.
U.S. Pat. No. 4,914,382 describes a sensor having a resistor between a tap adapter and earth (coupled to a capture instrument by a measuring transformer).
This solution allows measurement only of a signal correlated with the current that flows through the insulation, whereas quantities indicating the voltage applied to the insulation cannot be measured.
Besides this limitation, this technical solution has some drawbacks linked to the presence of the inductive coupling (that is, the measuring transformer), including the risk of introducing unwanted displacements, thus reducing measuring precision because of the additional impedance devices used, and narrowing the sensor's detection bandwidth.
U.S. Pat. No. 6,927,562 describes a sensor comprising a reference capacitor connected across the tap adapter and earth. Thus, a measurement of the voltage is taken at the capacitor electrode connected to the tap adapter, for detecting a signal indicating the voltage applied to the reference capacitor.
This solution, too, however, is not free of drawbacks in that it allows measurement only of a signal correlated with the voltage along the insulation whereas it does not allow detection of any signal indicating the current travelling in the insulation.
U.S. Pat. No. 4,757,263 describes a sensor equipped with a capacitive divider for detecting a signal indicating the voltage applied to the reference capacitor. Further, the device of U.S. Pat. No. 4,757,263 is configured to make a comparison with a reference source.
The disadvantage of this device, however, is that does not allow detection of any signal indicating the current flowing through the insulation.
It should be noted that often, in the field of devices for detecting signals correlated with the tan delta in a medium- or high-voltage insulator, the need is also felt to maximize the passband so as to detect a signal useful both for estimating the tan delta (in that case it is necessary to detect components in low frequency) and for detecting current pulses correlated with partial discharges, if any, present in the insulation (in that case it is necessary to detect components in high frequency).
In effect, it is known that estimating the loss factor and analysing the partial discharges are two different and, in certain respects, complementary techniques of diagnosing an insulation system and in many cases it is desirable to use both techniques in combination.
In light of this, it should be noted that U.S. Pat. No. 6,433,557 describes a circuit for picking up the signal from the tap adapter and comprising a reference capacitor connected across the tap adapter and earth, and an inductive coupling (that is, a signal transformer) coupled to the circuit leg formed by the reference capacitor.
This system is designed to measure simultaneously a signal indicating a voltage applied to the insulator (having only low frequency components) and a signal indicating high-frequency current pulses).
This solution, however, has the disadvantage of not allowing detection of a signal indicating the current flowing in the insulation and having low-frequency components useful for the tan delta.
This circuit also has the disadvantage of requiring the presence of the signal transformer (that is, of an inductive coupling), leading to problems of displacement, use of additional impedance devices and bandwidth limitations (similarly to what is stated above in connection with U.S. Pat. No. 4,914,382.
More generally, it should be noted that the above mentioned devices have the following limitations.
With these devices it is not possible to directly detect a set of data sufficient to directly derive (that is, estimate by calculation) the tan delta. In effect, it should be noted that to perform a true volt-amp measurement, it would be necessary to measure the high voltage applied to the insulation, but such a measurement is not performed because of the excessive risks and drawbacks consequent upon the high level of the voltage. involved (so much so that the tap adapter is used).
In light of this, all the prior art systems are unable to provide significant data for estimating the tan delta based on values measured at a given point in time; thus, the data provided by the prior art devices mentioned above are used for relative processing, that is to say, for comparison with previously detected data (time-trend analysis) or for comparisons between the data measured in the different phases of a poly-phase transformer.
From patent document JP56112662A it is known a measuring apparatus for measuring the loss factor of capacity element. In this apparatus a current/voltage converter is directly connected to the measured element (which, in turn, is connected to a voltage source); indeed, said apparatus does not comprise any reference capacitor.
Therefore, the apparatus of JP56112662A could not be used to measure the loss factor of a medium or high voltage capacitor, where the capacitor itself is accessible through a tap adapter and the voltage source is not accessible.